Acousto-optic devices usually consist of an electrically driven piezo-electric transducer bonded to an acousto-optic media. An acoustic strain wave, provided when the transducer is electrically addressed, modulates the refractive index of the acousto-optic media which in turn diffracts light in a preferred direction when light incident on the acousto-optic media is near the Bragg angle. The acousto-optic media acts as a grating, the grating period being equal to the wavelength of the sound wave in the media.
Such acousto-optic devices have applications in optical memory systems, acoustic delay lines, light modulators, and printing and display systems. In the latter applications, the acousto-optic device is utilized to diffract a high intensity light beam such that it sweeps across a light sensitive surface or is directed to selected spots on the light sensitive surface. Such light diffraction is conventionally achieved by utilizing a single transducer and simultaneously supplying signals of different frequencies to the transducer. Since the different frequencies of the supplied signals cause the acousto-optic media to have a multiplicity of sound waves with different grating periods, a light beam incident on the acousto-optic media will ideally be diffracted at angles corresponding in number to the number of supplied frequencies whereby a number of light spots corresponding to the number of supplied frequencies is provided at the light sensitive medium.
In such multi-frequency signal systems, the frequencies of the signals utilized must be carefully selected so that beat frequency signals resulting from the sum and differences of the supplied signals does not cause light to be diffracted out of the acousto-optic media at angles that will produce undesired light spots on the light sensitive medium. If the number of desired light spots is in the neighborhood of 10-20, the beat frequency signals are easily avoided by proper frequency selection. However, proper frequency selection becomes a major problem when it is desired to diffract out a hundred or more light spots.